After harvesting but prior to processing, potatoes and other surface-sprouting edible produce such as sweet potatoes, yams, beets, carrots, turnips, rutabagas and radishes are often stored for appreciable periods of time. During such storage sprouting often occurs, frequently destroying or impairing the quality and processing value of the produce.
To inhibit such sprouting and prevent bacterial decomposition, such produce is normally stored at cool temperatures, e.g., in the case of potatoes preferably between about 42.degree. and 50.degree. F., although temperatures outside this range may also be used, and a sprout-inhibiting chemical agent, preferably isopropyl N-3 chlorophenylcarbamate (CIPC), is periodically applied to the surfaces of the produce by circulation of an aerosol of a solution of CIPC in a harmless organic solvent such as a vegetable oil. A method for the use of this chemical agent is disclosed in Plant U.S. Pat. No. 3,128,170, which teaches application of an aerosol of isopropyl N-3 chlorophenylcarbamate dissolved in a water soluble organic solvent and atomized into liquid droplets of 1-10 micron diameter.
It has previously been considered a problem of distribution of the CIPC that, in a technically pure form, CIPC is a crystalline solid at normal storehouse temperatures. To solve this problem, prior methods such as that disclosed by the foregoing Plant patent have taught that the chemical should be dissolved in a solvent which can carry the CIPC in solution during formation of an aerosol, yet which will not adversely affect the stored produce. It is now most common for the CIPC to be carried by hygroscopic organic solvents such as vegetable oil, isopropyl alcohol, ethylene glycol or propylene glycol. The use of such solvents has also been necessary to reduce the normally high viscosity of the CIPC to the point where the chemical may be broken down into extremely small aerosol particles by previously available mechanical methods. The small droplet size is particularly necessary to permit the CIPC to be carried by air circulation systems within a storehouse without excessive harmful and wasteful deposition of the solvent on storehouse and air circulation system surfaces, and to provide even distribution of the CIPC over all the exterior surfaces of stacked produce. The maximum droplet diameter is about 10 microns for distribution without undesired deposition in the distribution conduits and the storehouse. Droplet diameters of 5 microns or less are preferred, since smaller droplets are more uniformly distributed by circulation of the aerosol through stacked produce.
What has not previously been recognized is that it is advantageous for the minute droplets of CIPC solution to crystallize while suspended in air, since the crystals are less likely than the liquid droplets to adhere to one another and agglomerate, thus inhibiting distribution throughout the produce, or to adhere to distribution ducts and storehouse walls. Use of a substantial amount of solvent, as taught for example by Plant, inhibits crystallization, thereby encouraging agglomeration and deposit of the CIPC solution on air ducts and storehouse surfaces, thereby obstructing air distribution duct work, wasting sprout-inhibiting chemicals, and creating a fire hazard. Plant, however, is forced to use a substantial amount of solvent in order to reduce the viscosity to achieve sufficiently small droplet size for uniform distribution of the sprout-inhibiting chemical, using the high-speed disc type of atomizing method and apparatus disclosed. This high-speed rotating disc method, which is capable of producing droplets small enough for distribution only if sufficient solvent is used, is also limited as to its rate of aerosol production and is accompanied by production and circulation of a considerable proportion of droplets of larger size than is desirable.
Partly because of some of the foregoing deficiencies of methods such as that disclosed by Plant, the presently most common method of forming the aerosol is atomization of a CIPC and solvent solution by a thermal fogger, a process which involves combustion of the solution in compressed air, accompanied by rapid expansion of the products of combustion. The thermal fogger operates similarly to a rocket engine, and includes a pressure chamber containing compressed air and a glowplug located nearby an exit nozzle in the pressure chamber. The sprout-inhibiting solution is introduced into the compressed air near the glowplug, which causes ignition due to the combustibility of the solution. As the burning solution expands through the exit nozzle, most of the solvent is burned and rapid expansion atomizes the mixture into a very fine mist. The mist produced has a very desirable range of sizes of particles, including only about 25% by weight in the range greater than 5 microns diameter and about 60% by weight in the range smaller than 1 micron in diameter.
A major disadvantage of use of the thermal fogger, however, is that it heats the CIPC to temperatures in the range of 700.degree. to 900.degree. F. and above. It has been discovered by the present inventors that, at temperatures above about 250.degree. F., the CIPC decomposes chemically. In fact, the predominant compound found in chromatographic analysis of a thermal fogger-produced mist was not CIPC but M-chloro aniline. Additional breakdown products were also produced, some of which are as yet unidentified by ordinary gas chromotograph methods. The result of the thermal fog-producing process is decomposition of up to 80% of the sprout inhibiting chemical; that is, of the portion of the resultant fog which can be identified as CIPC or products of its decomposition, as little as 20% is CIPC. Still more CIPC may be further decomposed to carbon, carbon oxides, etc. not traceable to the original CIPC.
As particle size decreases, the relative amount of decomposition apparently increases, with the result that the smaller particles contain a much smaller proportion of the sprout inhibiting CIPC than the larger ones. Therefore, although the overall distribution of the particles produced by the thermal fogger is quite impressive, the distribution of droplets of the non-decomposed sprout inhibiting chemical is far less desirable. A very significant proportion of the CIPC is in the form of the relatively large droplets which tend to drop from air suspension too quickly to be effectively circulated throughout the stored produce. Because of the foregoing chemical decomposition problems characteristic of the thermal fogger method of aerosol production, an excessively large amount of the chemical is needed to provide minimum concentration of the sprout-inhibiting chemical at some points within the stacks of stored produce, while unnecessarily high concentrations are achieved at other points. High temperatures encountered in the thermal fogging system also tend to warm the produce in storage to an undesirably high temperature during application of the chemical agent, which is conducive to growth of bacteria which may cause spoilage of the stored potatoes.
A method and device for producing a dry vapor, i.e., an aerosol composed of suspended droplets, predominantly less than 1 micron in diameter, is disclosed in Clark U.S. Pat. No. 3,860,401. A liquid spray aspirator nozzle is used by Clark to produce droplets of a relatively non-viscous liquid constituent of an aerosol for treating air, and particularly for controlling odor in hospitals. The droplets are mixed with air within a vertically extending centrifugal vortex separator. The Clark apparatus, however, has no apparent means for sufficiently atomizing oily or viscous liquids, such as CIPC, which is difficult to separate into extremely small particles without substantial amounts of solvent which inhibit crystallization of the CIPC. The Clark apparatus also appears to be a low-volume atomizer, and thus inappropriate for application of a chemical agent to a large quantity of stored produce.
Although it is known that production of smaller aerosol droplets of sprout-inhibiting chemical solutions ensures more uniform distribution of the chemical through stacked produce, and also helps to reduce the amount of the chemical solution deposited uselessly on storehouse and/or air conduit surfaces, an efficient method and apparatus of producing such small particles without requiring a substantial amount of solvent and at low enough temperatures to prevent significant decomposition of the chemical has not previously been known.
What is needed, therefore, is an apparatus for efficiently treating stored produce with a sprout-inhibiting agent in the form of an aerosol having droplets whose diameters are less than about 10 microns, and preferably less than 5 microns, at temperatures low enough to prevent any significant chemical decomposition of the sprout-inhibiting agent and without requiring the use of solvents in amounts which substantially inhibit crystallization of the aerosol droplets when exposed to storehouse ambient temperature.